Flash EEPROMs are programmed or erased by injecting electrons into the floating gate isolated between the control gate (or program gate) and the substrate or ejecting them therefrom.
Generally, in the NOR-type EEPROM cell, injecting electrons is called program, wherein channel hot electron method is usually used. That is, a voltage of about 9 volt is applied to the control gate, a voltage of about 5 volt is applied to the drain and the well and the source are grounded. In this condition, hot carriers are generated around the drain and electrons thereof are thus moved toward the gate by means of the electric field formed by the gate voltage. The program time at this time is about 5 .mu.s.about.10 .mu.s.
However, when a single outside power supply (Vcc is 5 volt, 3.3 volt, 2 volt, etc.) is used, in order to generate a high voltage necessary in programming, a charge pumping method is used. Then, after the voltage becomes higher, it is necessary to keep a constant voltage. The above-mentioned charge pumping method will be explained in detail.
The program voltage control circuit in the conventional flash EEPROM for realizing the charge pumping will be explained by reference to FIGS. 1 and 2.
As shown in FIG. 1, the conventional program voltage control circuit comprises a charge pump 20 for performing a pumping operation depending on a program signal PGM and an outside clock HVOSC that are inputted from the outside and an adjusting unit 30 for adjusting the output voltage from the charge pump 20. Also, the adjusting unit 30 comprises a voltage divider 31 for dividing the voltage outputted from the charge pump 20; a reference voltage generator 32 for generating a reference voltage; and a comparator 33 for comparing the dividing voltage REGLEVEL divided by the voltage divider with the reference voltage REGREF generated from the reference voltage generator 32 to control a leak path 34 depending on the output thereof, as shown in FIG. 2.
The charge pump 20 and the adjusting unit 30 are enabled by the program signal PGM. The charge pump 20 starts the pumping operation according to the outside clock HVOSC to produce a pumping voltage VPPI.
The pumping voltage VPPI outputted from the charge pump 20 is inputted to the adjusting unit 30. The pumping voltage VPPI is divided by the voltage divider 31 and is then inputted to the comparator 33. At this time, the comparator 33 compares the divided voltage REGLEVEL divided by the voltage divider 31 with the reference voltage REGREF produced by the reference voltage generator 32.
As a result of the comparison, when the divided voltage REGLEVEL becomes higher than the reference voltage REGREF, the comparator outputs a high signal. Due to this high signal, the transistor is turned on, so that remaining charges can be discharged via the leak path 34.
FIGS. 3A and 3B are the results of simulation showing variation of the voltage and the current, respectively. From the drawings, it could be seen that the current consumption by the output voltage of the charge pump 20 is similar around 9 volt.
In other words, as the charge pump is operated in a same cycle when the voltage is raised or kept constant, there is a problem that the consumption power is large because a constant amount of current is always consumed from the beginning the program to the end.